Inhaled silica has been compellingly linked to several human autoimmune diseases, including systemic lupus erythematosus and ANCA vasculitis that destroy kidneys, lungs, and other organs. However, little is known about the mechanism of autoimmune induction or the role of genetic susceptibility. Autoantibodies are prominent in these disorders and key mediators of tissue injury. The experiments proposed here test the overarching hypothesis that the silica-exposed lung creates a microenvironment that alters autoimmune cell regulation in genetically susceptible individuals. We further propose that this breach in B cell tolerance occurs through the intermediary of pulmonary tertiary lymphoid structures or iBALT that promote survival and activation of autoreactive lymphocytes, and that the human HLA DRB1*1501 risk allele and Toll-like receptor ligand co-exposure contribute to this breach. This hypothesis can be tested using in vivo models that permit complex and dynamic immune cell interactions at the environment/lung interface and that are amenable to mechanistic dissection. We propose three Specific Aims supported by extensive preliminary data and established cross-disciplinary collaborations. Specific Aim 1 tests the hypothesis that silica- induced iBALT is a major site for loss of B cell tolerance, and that the extent and nature of iBALT formation and defective tolerance varies according to genetic susceptibility. We will measure recruitment and activation of autoreactive B cells within iBALT and secondary lymphoid organs of silica-exposed subjects using flow cytometry, immunoassay, cell culture, and microdissection. This aim is possible using a unique and experimentally tractable murine model system developed in our lab, in which an autoantibody transgene serves as a reliable reporter to track autoreactive cells and monitor well-defined tolerance phenotypes within the context of genetically distinct B6 and autoimmune MRL, NZB, and BXSB strains that collectively mirror human lupus genetic heterogeneity. Specific Aim 2 tests the role of a potent human autoimmune risk allele, HLA class II DRB1*1501, in silica-induced iBALT induction and proteinase 3 (PR3)- ANCA vasculitis. This aim uses two novel humanized models that replace murine class II molecules with human class II DR2 (DRA1/DRB1*1501). We will measure silica exposure impact on autoreactive cell recruitment and tolerance using DR2+ B6 autoAb Tg mice, and on anti-PR3 autoreactivity and vasculitis using dual humanized Hu-HSC mice expressing DR2 both in thymus and on human immune cells and subject to PR3 immunization or PR3-ANCA infusion. Specific Aim 3 tests the capacity of silica to break tolerance to myeloperoxidase (MPO) or to modify anti-MPO immunity and MPO-ANCA vasculitis. This aim takes advantage of MPO immunoreactivity and disease-susceptibility in MRL and MPO-deficient B6 models. Ultimately, insight into mechanisms of silica-controlled autoimmunity will identify new targets and new routes for therapeutic intervention to arrest injury and prevent relapses in patients with autoimmune disease.